Several genomic clones that hybridized to a CDNA clone for HLA (human histocompatibility) were isolated from pig and mouse genomic lambda libraries. Mouse tk cells were then cotransformed with herpes simplex virus thymidine kinase gene and several of these clones that putatively encoded for histocompatibility antigens. Several of these cotransformants appear to coexpress thyumidine kinase and proteins that can be identified by the use of antisera to known histocompatibility antigens. Other genomic clones cannot be identified with such antisera; some probably cannot express any histocompatibility antigens and others probably express antigens unknown at the present time. Among those that express known antigens we have identified genomic clones encoding for an SLA (porcine major histocompatibility) antigen and those encoding for mouse major histocompatibility antigens H-2L and H-2D.